Metallic mirrors have numerous applications in the field of optics. Typically, mirror surfaces are formed by applying a metallic coating to a substrate composed of another metal or of another material such as glass. When subjected to temperature variations, such as in various space and military applications, differential rates of expansion and of contraction of the mirror components lead to dimensional instability of the mirror surface. One solution has been to polish the surface of substrates composed of a single metal or metal alloy, which obviates problems caused by differences in thermal properties, and provides the substrate with the mechanical strength and rigidity of metals. Aluminum is a preferred metal due to its lightness, low cost, and compatibility with conventional surface forming processes. The performance of such mirrors in optics applications depends strongly on the surface needing to be highly uniform to minimize light scattering caused by surface irregularities.
Surface roughness parameters relevant to optical performance include not only average surface roughness (Ra) or root mean square surface roughness (Rq), but also importantly parameters including Rmax and/or Rz. Rmax is the largest peak-to-valley height in a given sampling region wherein the peak represents a high spot on the surface and the valley represents the depth of a scratch on the surface. Rz is an average value for Rmax measured in several distinct sampling regions. Not only must average surface roughness be low for good optical performance, but also Rz, Rmax or related parameters must be low to minimize light scattering.
Existing methods for mirror fabrication attempt to remedy the drawbacks resulting from imperfectly formed metal surfaces by electroplating a layer of electroless nickel onto the mirror substrate and then using optical techniques to polish the second metal layer to provide surfaces exhibiting reduced light scattering. However, because again such mirrors consist of different metals, bimetallic stresses at temperatures differing from room temperature compromise mechanical and optical stability of the mirrors. Further, fabrication of the second metal layer adds significant complexity and expense to the manufacturing process.
Highly polished aluminum surfaces have additional applications in the field of solar cell fabrication. Mirror arrays are frequently used to concentrate solar radiation onto photovoltaic solar cells to improve conversion efficiency. Since diffraction of light by surface irregularities on the mirror surfaces results in lowered efficiencies in conversion of incident solar radiation to electric power, improved methods for economically producing highly reflective surfaces composed of aluminum can facilitate development of solar cell technology.
Two methods are commonly used to polish aluminum surfaces. In the field of optics, single-point diamond turning has been used for many years to produce aluminum mirrors useful for reflecting infrared (e.g., long wavelength) light. In single-point diamond turning, an aluminum substrate is rotated while in contact with a precisely positioned diamond cutting tool. The diamond cutting tool “peels” a very thin layer of aluminum from the surface of the substrate to form a surface having a precisely defined geometry. However, the diamond cutting tool produces microscopic grooves which compromise optical performance due to light scattering, particularly at shorter wavelengths. Further, single-point diamond turning is an expensive and highly time-consuming process suitable only for low volume production of specialized optical components.
The most commonly used process for polishing aluminum substrates is lapping. In lapping, a slurry of abrasive particles, typically aluminum oxide or silicon carbide in a carrier of water or an oil, is used to polish an aluminum surface by moving a polishing surface known as a lapp relative to the aluminum surface with the abrasive slurry therebetween, to abrade the surface by mechanical action of the abrasive. However, lapping generates shavings of aluminum which tend to produce microscratches on the polished surface, leading to unacceptable surface defectivity for optical applications.
Chemical-mechanical polishing or planarization (CMP) has long been used in the electronics industry to polish or planarize the surface of memory or rigid disks. Typically, memory or rigid disks comprise an aluminum substrate coated with a first layer of nickel-phosphorus. The nickel-phosphorus layer is frequently planarized by a CMP process to reduce surface waviness and roughness prior to coating with a magnetic layer, such as cobalt-phosphorus. The nickel-phosphorus layer has a highly homogeneous microstructure and a particular chemistry which differs considerably from aluminum and aluminum alloys, which have a different surface chemistry than nickel-phosphorus and an inhomogeneous microstructure comprising crystallites dispersed throughout a matrix. Further, CMP produces microscratches and leaves imbedded abrasive particles on the substrate surface, which defects cannot be tolerated in optical applications.
Thus, there remains a need for efficient and economical methods of polishing aluminum surfaces to exacting standards of surface roughness suitable for diffraction-free reflectance of light in the visible and ultraviolet ranges. The invention provides such a method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.